102
Remanufacturing and Advanced Machining
dozen seconds in the case of induction heating to dozens of hours in flame furnaces.
Depending on chemical composition, technological coatings should be able to melt
quickly and form protective films, as well as maintain their viscosity in long-term
heating.
In hot forming processes, a coating should display a required viscosity and form
a thermoresistant technological lubricant. It should not interrupt plastic deformation
of the hot metal, sustain high pressure and temperature, and make it easier to remove
the workpiece out of the die. Higher levels of protection are achieved at better wettability and adhesion to the component surface. Coatings are often expected to be
self-removable and crush out or peel off after a technological operation.
Moreover, a technological coating should be non-toxic, incombustible, and easily
available, and should ensure its low consumption per unit area of a protected metal
surface.
2.2.2 sTarTing maTerials for coaTings
For protective technological coatings, a variety of materials can be used of different
natures, chemical compositions, and properties. Considering the wide range of the
hot working processes in various temperature and time conditions, as well as effects
of furnace design on heating rates, the number of different coating materials should
be very large. These may be categorized according to their nature, since ceramics
and glasses are radically different materials than metals, but are close cousins to
each other, though ceramics are crystalline, while glasses are amorphous (Messler,
2006). Hence, glasses progressively soften upon heating and never melt, while
ceramics almost always exhibit high melting temperatures and/or thermal stability.
Thus, these three categories may be applied to classification of coatings:
1. Glass-like materials and glasses
2. Ceramics and pure oxides (Al 2 O 3 , ZrO 2 , MgO, Cr 2 O 3 )
3. Metals and intermetallic compounds, carbides, nitrides, etc.
For glass processing, the starting material is a glass batch, which is formulated from
a variety of materials, some mined from the earth and used with only a few preparatory steps, and some more refined, such as metal oxide powders (Francis, 2016).
Materials in a glass batch perform different functions, either supply a metal oxide for
glass composition or are additives that help produce a uniform melt. Glass powder
or frit is considered a glass starting material and is prepared by quenching a uniform
glass melt. A glass batch is heated in a furnace and converted into a homogeneous
melt. Quenching a glass melt from a high temperature into water or between metal
rollers results in thermal shock, so that glass is fractured into fragments, which further can be broken down using ball milling to create a frit of a desired particle
size. When heated, a powdery glass batch is converted to a homogeneous glass melt
(Francis, 2016).
Vitreous coatings are made of enamel frits that consist of alkaline aluminaborosilicate to which other inorganic substances may be added to provide desirable
Remanufacturing and Advanced Machining
dozen seconds in the case of induction heating to dozens of hours in flame furnaces.
Depending on chemical composition, technological coatings should be able to melt
quickly and form protective films, as well as maintain their viscosity in long-term
heating.
In hot forming processes, a coating should display a required viscosity and form
a thermoresistant technological lubricant. It should not interrupt plastic deformation
of the hot metal, sustain high pressure and temperature, and make it easier to remove
the workpiece out of the die. Higher levels of protection are achieved at better wettability and adhesion to the component surface. Coatings are often expected to be
self-removable and crush out or peel off after a technological operation.
Moreover, a technological coating should be non-toxic, incombustible, and easily
available, and should ensure its low consumption per unit area of a protected metal
surface.
2.2.2 sTarTing maTerials for coaTings
For protective technological coatings, a variety of materials can be used of different
natures, chemical compositions, and properties. Considering the wide range of the
hot working processes in various temperature and time conditions, as well as effects
of furnace design on heating rates, the number of different coating materials should
be very large. These may be categorized according to their nature, since ceramics
and glasses are radically different materials than metals, but are close cousins to
each other, though ceramics are crystalline, while glasses are amorphous (Messler,
2006). Hence, glasses progressively soften upon heating and never melt, while
ceramics almost always exhibit high melting temperatures and/or thermal stability.
Thus, these three categories may be applied to classification of coatings:
1. Glass-like materials and glasses
2. Ceramics and pure oxides (Al 2 O 3 , ZrO 2 , MgO, Cr 2 O 3 )
3. Metals and intermetallic compounds, carbides, nitrides, etc.
For glass processing, the starting material is a glass batch, which is formulated from
a variety of materials, some mined from the earth and used with only a few preparatory steps, and some more refined, such as metal oxide powders (Francis, 2016).
Materials in a glass batch perform different functions, either supply a metal oxide for
glass composition or are additives that help produce a uniform melt. Glass powder
or frit is considered a glass starting material and is prepared by quenching a uniform
glass melt. A glass batch is heated in a furnace and converted into a homogeneous
melt. Quenching a glass melt from a high temperature into water or between metal
rollers results in thermal shock, so that glass is fractured into fragments, which further can be broken down using ball milling to create a frit of a desired particle
size. When heated, a powdery glass batch is converted to a homogeneous glass melt
(Francis, 2016).
Vitreous coatings are made of enamel frits that consist of alkaline aluminaborosilicate to which other inorganic substances may be added to provide desirable